1. Field of the Invention
This invention relates to integrated circuit packaging. More particularly, this invention relates to an integrated circuit package having a lead frame compensated for thermal mismatch between the lead frame and the integrated circuit die.
2. Background Art
Conventionally, an integrated circuit chip is packaged by bonding the silicon chip to the center, die paddle portion, of a lead frame constructed of a nickel-steel alloy such as Alloy 42 which is selected for good electrical conductivity as well as thermal expansion characteristics matching that of silicon. However, with continued shrinkage of size of integrated circuit structures and consequent dense packing of the devices constituting the integrated circuit structure the efficient removal of heat generated by the integrated circuit structure has become more important. This, in turn, has lead to the use of copper lead frames due to the excellent heat and electrical conductivity properties of copper.
However, despite the improved heat dissipation characteristics of an integrated circuit package constructed using a copper lead frame, the difference between the thermal expansion coefficients of copper and silicon has necessitated the use of thicker and, sometimes, more flexible bonding agents, such as silver-filled epoxy resins, to bond the silicon die to the copper lead frame to prevent cracking of the silicon die as heat is generated by the package during normal operation. The thickness, however, of such a bonding agent is not easily controlled. Furthermore, the use of a thick and flexible bonding agent can result in tilting of the die, i.e., non-parallel spacing between the die and the lead frame, making lead attachment more difficult, particularly with automated equipment.
It would, therefore, be desirable to provide for thermal compensation between a silicon integrated circuit die and a lead frame having a different thermal coefficient of expansion by relieving the stress inside the lead frame rather than through the die attachment means.